The present invention relates to a radiation image detecting apparatus advantageously employable for a radiation image forming method. The apparatus comprises a combination of a phosphor screen and a two-dimensionally extended photo-detector.
In a variety of radiography such as X-ray radiography for medical diagnosis or non-destructive inspection, a radiation image forming method (i.e., radiographic photography) utilizing a combination of a radiographic film and a radiographic intensifying screen is generally used. The radiographic intensifying screen absorbs radiation such as X-rays, and emits light in a visible region. Hence, the radiographic film is exposed to both of the emitted visible light and the radiation to form a radiation image on the film. The intensifying screen usually comprises a support, a phosphor layer and a protective film overlaid in order. The phosphor layer usually comprises a binder and phosphor particles dispersed therein, but it may consist of agglomerated phosphor with no binder. The phosphor layer containing no binder can be formed by deposition process or firing process. Further, in order to prevent diffusion of the visible light (luminescence) emitted from the phosphor in the phosphor layer, a partition that divides the phosphor layer on its plane to give small sections may be provided.
Recently, in the field of digital radiography, a radiation image forming method utilizing a radiation image detecting apparatus which comprises a combination of a phosphor screen sectioned in the form of pixels and a two-dimensionally extended photo-detector has been proposed. This method comprises the steps of converting radiation such as X-rays into visible light by means of the phosphor screen, detecting the visible light by means of the photo-detector to obtain electric signals of image information, and transferring the signals to a proper image-reproducing means so as to form a radiation image. For the two-dimensionally extended photo-detector, devices such as photo-diodes, photo-transistors, photo-electroconductive elements and charge-coupled devices (CCD) are used.
U.S. Pat. No. 5,153,438 discloses a combination of a phosphor screen having been sectioned to give a large number of pixels and an array of photo-detectors. Each pixel of the phosphor screen is designed to have the same size and shape as those of each photo-receiving cell of the photo-detector array, so that each pixel may correspond to each cell by a one to one mode. The phosphor screen is formed in the form of pixels by deposition process, notching process, abrasion process, or chemical etching process. The spaces between the pixels may be filled with a light-reflecting material such as titanium dioxide.
Japanese Patent Provisional Publication No. 7-198852 discloses a constitution in which a photo-fiber plate is provided between the phosphor screen in the form of pixels and the array of photo-detectors. In this constitution, each detector of the array corresponds to photo-fibers of the plate and pixels of the screen by a one to one or one to more mode. The spaces between the pixels may be filled with a material having a refractive index lower than that of the material of pixels (e.g., phosphor having different composition from that in the pixels) so that the emitted light may be enclosed in each pixel.
It is desired that even a phosphor screen in the form of pixels for the use in the aforementioned radiation image detecting apparatus give a radiation image of high quality (particularly, high sharpness for high resolution) with a small dosage of radiation.
It is an object of the present invention to provide a radiation image detecting apparatus giving a radiation image of high quality, particularly high sharpness.
The inventors have studied about the phosphor screen of the apparatus, and found that a specifically designed phosphor screen gives a radiation image of high quality. The phosphor screen employed in the invention comprises a partition that divides the screen on its plane to give small sections and phosphor-incorporated areas that are sectioned with the partition to give a large number of pixels. In the phosphor screen in the form of pixels, the partition is designed to scatter luminescence emitted from the phosphor with a short scattering length, so as to exhibit such a high reflectance enough to effectively prevent planar diffusion of the luminescence. At the same time, the partition is also designed to absorb the luminescence with a long absorption length, so as to exhibit such an absorbance enough to avoid lowering the amount of the observed luminescence. On the other hand, the phosphor-incorporated areas are designed to scatter and absorb the luminescence with a long scattering length and a long absorption length, respectively. Since the phosphor-incorporated areas are thus made to have a high transmittance and a low absorbance to the luminescence, the luminescence emitted from the phosphor near the surface exposed to radiation (i.e., the luminescence emitted from the phosphor far from the photo-detector) can be effectively collected.
The present invention resides in a radiation image detecting apparatus comprising a phosphor screen that converts radiation into visible light and a two-dimensionally extended photo-detector placed on one surface of the screen, wherein the phosphor screen comprises a partition that divides the screen on its plane to give small sections and phosphor-incorporated areas that are sectioned with the partition, the phosphor-incorporated areas and the partition scattering the visible light with scattering lengths of 20 to 200 xcexcm and 0.05 to 20 xcexcm, respectively, providing that the ratio between them is not less than 3.0, and the phosphor-incorporated areas and the partition absorbing the visible light (emitted from the phosphor) with an absorption length of not less than 1,000 xcexcm.
The scattering length indicates a mean distance in which light travels straight until it is scattered, and hence a small value means that the light is highly scattered. Also, the absorption length indicates a mean distance in which light travels straight until it is absorbed, and hence a large value means that the light is hardly absorbed. The scattering length and the absorption length can be obtained by measuring the thickness xe2x80x9cdxe2x80x9d and the transmittance xe2x80x9cTxe2x80x9d of the film sample and calculating from them according to the following formula (1) based on Kubeluka-Munk theory.
T(d)=(xcex7xe2x88x92"xgr")/(xcex7xc2x7exp(xcex3d)xe2x88x92"xgr"xc2x7exp(xe2x88x92xcex3d))xe2x80x83xe2x80x83Formula (1)
in which
xcex32=xcex2(xcex2+2xcex1)
"xgr"=(xcex1+xcex2xe2x88x92xcex3)/xcex1
xcex7=(xcex1+xcex2+xcex3)/xcex1
The scattering length 1/xcex1 and the absorption length 1/xcex2 can be obtained by incorporating the measured d and T values into the formula (1) and optimizing, for example, using a least squares method.
Preferred embodiments of the phosphor screen used in the radiation image detecting apparatus of the invention are described below.
(1) The phosphor screen in which the phosphor-incorporated areas contain at least a phosphor and a binder.
(2) The phosphor screen in which the phosphor-incorporated areas contain the phosphor in a volume ratio of 40 to 95% and has a void volume of 0 to 20%.
(3) The phosphor screen in which the partition contains at least low photo-absorbing fine particles and a polymer material.
(4) The phosphor screen in which the partition contains the low photo-absorbing fine particles in a volume ratio of 30 to 90%.
(5) The phosphor screen in which the low-photo-absorbing fine particles have a mean size in the range of 0.01 to 5.0 xcexcm.
(6) The phosphor screen in which the low photo-absorbing fine particles are alumina fine particles.
(7) The phosphor screen in which the partition contains voids.
(8) The phosphor screen in which the partition contains a void volume of 10 to 70%.
(9) The phosphor screen in which the low photo-absorbing fine particles and the voids have refractive indexes between which ratio is in the range of 1.1 to 3.0 (in terms of former/latter).
(10) The phosphor screen in which the partition further contains a phosphor.
(11) The phosphor screen in which the partition further contains a material that absorbs visible light.
(12) The phosphor screen having a thickness of 50 to 1,500 xcexcm.